1. Field of the Invention
This invention relates generally to aerobic wastewater treatment systems and methods and, more particularly, to aerobic wastewater treatment systems and methods which utilize a moving stream of mixed liquor in a retention basin, such as an oxidation ditch.
2. Prior Art
In many aerobic wastewater treatment systems wastewater flows into a retention basin, such as an oxidation ditch, where the wastewater is retained as mixed liquor. In a conventional oxidation ditch system, a continuous channel contains the flowing mixed liquor, which is aerated as it flows around the channel to provide oxygen for microorganisms which decompose the waste in the channel. Thus, as waste is decomposed in the basin, suspended solids form in the mixed liquor. Clarifiers are used to clarify the mixed liquor by settling the suspended solids and removing clarified wastewater from the system. Intrachannel clarifiers may be employed for this purpose. Examples of intrachannel clarifiers may be seen in Harold J. Beard's U.S. Pat. Nos. 4,362,625; 4,383,922; and 4,457,844. Maintenance of the proper biological balance in the basin is essential for proper operation of the system.
Various measures are used for maintenance of proper biological activity. One such measure is known as "sludge age". For a given concentration of waste entering the basin and a given concentration of waste within the basin, the sludge age can be calculated. Thus, the number of pounds of waste within the basin divided by the number of pounds of waste per day entering the basin will yield a sludge age in days. It has been found empirically that a sludge age of 23-25 days will yield excellent biological activity within the basin.
Since only clarified wastewater is taken from the system, the solids concentration within the basin increases as wastewater continues to flow into the basin, increasing the sludge age. Varying concentrations of wastewater entering the basin over time also vary the sludge age. The retention of solids in the basin requires that mixed liquor be wasted from the basin in order to maintain a desirable sludge age. Overwasting or underwasting can result in disastrous biological consequences. Prior art wasting was accomplished by observation of basin conditions and occasionally opening a waste valve or operating a waste pump, guessing at the amount of solids removed from the basin. Prior art wasting methods require expert system operators. Further, variations in operating conditions often result in unforeseen variations in sludge age, regardless of the skill of the operator.
An additional problem is created when wasting mixed liquor. Mixed liquor exiting a basin has a very high water content. Thus, in order to dispose of a small amount of solids, a very large volume of mixed liquor must be removed. The wasted mixed liquor must be processed in digesters or other similar sludge disposal systems.